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FreeBSD-EN-16:15.vmbus                                          Errata Notice
                                                          The FreeBSD Project

Topic:          Better handle the GPADL setup failure in Hyper-V

Category:       core
Module:         vmbus
Announced:      2016-08-12
Credits:        Microsoft OSTC
Affects:        FreeBSD 10.3
Corrected:      2016-06-15 09:39:41 UTC (stable/10, 10.3-STABLE)
                2016-08-12 04:01:16 UTC (releng/10.3, 10.3-RELEASE-p7)

For general information regarding FreeBSD Errata Notices and Security
Advisories, including descriptions of the fields above, security
branches, and the following sections, please visit
<URL:https://security.FreeBSD.org/>.

I.   Background

FreeBSD guests on Hyper-V call hv_vmbus_post_message(), which uses hypercalls
to post various types of VMBus messages to the Hyper-V host.  One kind of the
messages is to setup GPADL (Guest Physical Addrss Description List), which
describes shared memory buffers between the host and the guest.

II.  Problem Description

Most often, hv_vmbus_post_message() does not fail.  However, it could fail
intermittently when GPADLs of large shared memory is to be established with
the host, such as on the hn(4) attach path, a GPADL of 15MB sendbuf is
created, for which lots of messages will be flooded to the host.

The host side tries to throttle the message rate by returning
HV_STATUS_INSUFFICIENT_BUFFERS.

Before this errata notice, several retries for failed messages would be
attempted, but the delay between each retry is too low, causing sporadic
message posting failure.  A larger delay (>=1ms) is now used between each
retry to fix the message posting failure.

III. Impact

Failures when Hyper-V network device driver are loaded and the device cannot
work would occur.

IV.  Workaround

No workaround is available, however FreeBSD virtual machines not running in
Hyper-V or Azure are unaffected.

V.   Solution

1) Upgrade your system to a supported FreeBSD stable or release / security
branch (releng) dated after the correction date, and reboot the system.

2) To update your system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install

Afterward, reboot the system.

3) To update your system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch https://security.FreeBSD.org/patches/EN-16:15/vmbus.patch
# fetch https://security.FreeBSD.org/patches/EN-16:15/vmbus.patch.asc
# gpg --verify vmbus.patch.asc

b) Apply the patch.  Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile your kernel as described in
<URL:https://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.

VI.  Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/10/                                                        r301924
releng/10.3/                                                      r303984
- -------------------------------------------------------------------------

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN --summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing NNNNNN with the revision number:

<URL:https://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-EN-16:15.vmbus.asc>
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